Isocyanates are a class of chemicals which are used in the production of a wide variety of chemical compounds. These chemical compounds are in turn incorporated into a vast number of products which are used in great quantities world-wide. Monoisocyanates are used as intermediates in the production of herbicides, crop protection agents, and antidiabetic pharmaceuticals, while long-chain aliphatic monoisocyanates are used for the surface treatment of textiles. Diisocyanates and polyisocyanates are intermediates in the manufacture of polyurethane materials. These materials include rigid foams for insulation, flexible foams for seating, and paints yielding durable finishes. In preparing these and other commercial products, manufacturers utilize the various species of isocyanates, both alone and in combination, in order to obtain the desired characteristics in the final product.
While isocyanates provide a great many benefits, their use is unfortunately accompanied by certain problems. One of the most serious of these is the effect of isocyanates on the human respiratory system. Upon inhalation, isocyanates act as respiratory irritants. Fortunately, in the short term, the symptoms resulting from isocyanate inhalation usually disappear after removal of the person from the contaminated environment. Repeated exposure to isocyanates over a prolonged period, however, can lead to progressive and permanent impairment of pulmonary function. This impairment manifests itself in the form of shortness of breath and increased stress on the heart. More seriously, a "sensitized" condition arises in approximately 5 percent of all persons exposed to isocyanates. In this condition, asthmatic symptoms present themselves almost immediately upon exposure to even relatively low concentrations of isocyanates, i.e., concentrations which do not affect those who are not sensitized to isocyanates.
Because of the serious adverse physiological effects associated with isocyanate inhalation, attention has been given to devising methods for the detection of isocyanates. Several of these methods are concerned with determining the concentration of particular airborne isocyanate species in an environment, while others are directed toward measuring the residual isocyanate monomer content in various isocyanate starting materials, e.g., bulk isocyanate prepolymers and the like. In addition, methods for determining the percentage of free isocyanate groups present in urethane-based polymers have also been developed. Although such methods are predominantly spectrophotometric or chromatography-based, polarography, potentiometry, dielectric constant determination, detector tubes, impregnated paper tapes, and coated piezoelectric crystals have also been used for this purpose. An overview of various methods presently available for the detection of isocyanate species in air is provided in Dharmarajan et al., "Recent Developments in the Sampling and Analysis of Isocyanates in Air, Sampling and Calibration for Atmospheric Instruments," Am. Soc'y for Testing & Materials, Spec. Tech. Pub. No. 957, pp. 190-202 (1987), and Purnell et al., "Methods for the Determination of Atmospheric Organic Isocyanates: A Review," Analyst, 110, 893-905 (1985).
While the detection and quantification of particular isocyanate species are important, it is also desirable that a method be able to detect and quantify the total number of isocyanate groups present in an environment, regardless of the particular isocyanate species which are present. This arises from the fact that the health risks mentioned previously may not occur only as a result of exposure to a single isocyanate species. In recognition of this, the United Kingdom has adopted an exposure standard for isocyanates which is based upon the total isocyanate groups present in an environment. Silk et al., Ann. Occup. Hyg., 27 (4), 333-39 (1983).
One particular method which could conceivably be used to determine the total isocyanate groups present in a sample is described in Marcali, "Microdetermination of Toluenediisocyanates in Atmosphere," Anal. Chem., 29 (4), 552-558 (1957). The method described in that article comprises initially hydrolyzing a toluenediisocyanate (TDI) monomer to prepare a derivative thereof, i.e., a toluenediamine (TDA). Diazotization of the TDA, and subsequent coupling of the stable diazo compound with N-1-naphthylethylenediamine, is then undertaken. This results in the production of a compound having a reddish-blue color, which compound may be measured spectrophotometrically to determine the isocyanate level.
This method was originally intended by its developers to enable the detection of trace quantities, i.e., down to 10-20 ppb, of particular isocyanate monomer species in air, namely toluenediisocyanates. However, in practice, it was discovered that the method was susceptible to interferences which subverted that goal. For example, it was found that TDA and any other aromatic amines present in the sample were also diazotized and bonded to the N-1-naphthylethylene diamine. As TDI is converted to TDA in the reaction scheme, TDA present in the air sample will result in a false positive reading for TDI. Moreover, the method possesses poor sensitivity (in the range of 20 ppb) relative to HPLC methods developed in the 1980's. Further, different isocyanates have different response factors. Accordingly, quantification of isocyanates can only be accomplished for those species for which the response factor is already known. In addition, mixtures of isocyanates for which the response factors are known cannot be accurately quantified without knowing their relative amounts. For example, 2,4-TDI and 2,6-TDI, which are typically found together in products, have different response factors. As such, obtaining an accurate measure of the total isocyanate species depends upon knowing the relative amount of each.
A method which has been used for detecting the total isocyanate presence in a sample involves derivatizing isocyanates by forming ureas therefrom using 1-(2methoxyphenyl)piperazine (MOPP): ##STR1##
The resulting ureas are then analyzed using high performance liquid chromatography (HPLC) equipped with ultraviolet (UV) and electrochemical (EC) detectors. Isocyanate-derived peaks are identified on the basis of their UV/EC response ratio and all such peaks are quantified using an isocyanate monomer standard. The total airborne isocyanate concentration is then calculated from the sum of all isocyanate-derived HPLC peaks as described in "Health and Safety Executive: MDHS 25, Methods for the Determination of Hazardous Substances: Organic Isocyanates in Air," Health & Safety Executive/Occupational Safety and Hygiene Laboratory (1987).
An evaluation of this method, however, has revealed that neither detector (UV or EC) response was found to be proportional to the number of derivatized isocyanate groups present in model urethane oligomers. See Streicher et al , . "Investigation of the Ability of MDHS 25 to Determine Urethane-Bound Isocyanate Groups," presented at the American Industrial Hygiene Conference and Exposition, Boston, Mass., May 30-June 5, 1992. This non-proportional response makes the method unreliable in terms of both its ability to correctly identify isocyanate species and inaccurate in its quantification of such species.
Another reaction scheme which has been considered in an effort to quantify the total isocyanates in a sample, e.g., air, involves passing the air through an impinger containing propanol under favorable conditions, wherein the isocyanate species reacts with propanol to yield their respective propyl carbamates. The excess propanol is then removed from the reacted mixture, the carbamate is subsequently hydrolyzed, and the resulting propanol is analyzed. The amount of propanol provides a quantification of the total isocyanates present. See Robertson, "Determination Of Total Isocyanate Concentrations In Air By Headspace Gas Chromatography," Section Paper of the Health & Safety Executive, Research & Laboratory Services Division (1986).
This methodology, however, suffers from at least three drawbacks. First, since the derivatizing reagent and the analyte are identical (propanol), the derivatizing reagent may introduce inaccuracies into the analysis if it is not completely removed after the formation of the carbamate. Secondly, the conditions required to regenerate the propanol from the carbamate are relatively harsh, and, thirdly, the rate of the hydrolysis reaction varies substantially with the various isocyanate species.
Yet another method utilizes tryptamine: ##STR2## as the derivatizing reagent for the isocyanate. After derivatization, HPLC with fluorescence and electrochemical detection in series is employed. See Wu et al., "Application of Tryptamine as a Derivatizing Reagent for the Determination of Airborne Isocyanates: Part 3. Evaluation of Total Isocyanates Analysis by High-Performance Liquid Chromatography With Fluorescence and Amperometric Detection," Analyst, 115, 801-807 (1990).
While this method provides a relatively higher degree of selective detection and a less-variable response factor than the method which uses 1-(2-methoxyphenyl)piperazine, there are certain aspects that could be improved upon. For example, a reagent which reacts with isocyanates faster than tryptamine would be desirable. This is because the more reactive the reagent is with an isocyanate, the smaller is the problem of losses of isocyanates to side reactions after the isocyanate is collected but prior to derivatization. Secondly, a reagent that provides a greater detector response than tryptamine would enable determination of the quantity of isocyanates at lower concentration levels. Finally, a reagent that yields derivatized isocyanates whose detector responses vary less than those derived from tryptamine would enable a more accurate identification and quantification of isocyanate species.
Other reagents have also been utilized to derivatize isocyanates, rendering them analyzable. For example, 9-(methylaminomethyl)anthracene (MAMA): ##STR3## and 1-(2-pyridyl) piperazine: ##STR4## have been used in an effort to determine the presence of particular isocyanate species present in a sample as opposed to the total species present therein. With respect to MAMA, this reagent could conceivably be used to detect total isocyanate species present in a sample. However, in practice, the response of the derivatized isocyanate formed thereby would be influenced by the physical proximity of the isocyanate functionality, thus introducing error into attempts to quantify the total isocyanate presence. In addition, it would be advantageous if a means were provided by which isocyanates could be derivatized at a rate which is faster than that provided by MAMA.
However, it is believed that MAMA and 1-(2-pyridyl)piperazine have not been used to determine the total amount of isocyanates present in a sample. Moreover, and as regarding MOPP, it would be advantageous if a means were available whereby certain properties, e.g., the rate of reaction with isocyanates and detectability, could be improved over that provided by MOPP.
Thus, there exists a need for a method which will provide a relatively safe and simple means for the quantitative detection of all isocyanate species present in a sample at low concentrations. Moreover, there exists a need for a method which minimizes the problems associated with the detection of isocyanates due to their instability.
It is therefore an object of the present invention to provide a method which is able to derivatize an isocyanate functionality and thereby provide a means by which the presence of low amounts of isocyanates can be detected both readily and with a high degree of accuracy. Another object is to provide a means for quantifying the total presence of isocyanate species in a sample, regardless of the particular species present in the sample. A related object of the present invention is to provide a method which provides for the derivatization of isocyanates at a relatively rapid rate as compared to known methods. A further object of the present invention is to provide a method wherein the reagents are analytically distinguishable from the reaction product which is analyzed to determine the presence of isocyanates, i.e., the analyte.
These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.